`spacerWidth` setting missing in pie charts

[Jolanrensen]

Lets-plot supports the spacer_width setting in pie charts to define, well, the spacer width. This can prevent issues like this:

Default:

With spacer_width set to 0.1:

Kandy can support it, since setting it manually like this works:

private val spacerWidthAes = Aes("spacer_width")
var PieContext.spacerWidth: Double?
    get() = (bindingCollector.settings[spacerWidthAes] as? NonPositionalSetting<Double>)?.value
    set(value) {
        bindingCollector.settings[spacerWidthAes] = NonPositionalSetting(spacerWidthAes, value)
    }